New norlignan enantiomers from the fruit of Crataegus pinnatifida with neuroprotective activities

(±)-Crataegusnorin A (1a/1b) and B (2a/2b), two pairs of rare 8,9′-epoxy-type norlignan enantiomers featuring a γ-butyrolactone ring, were isolated from the fruit of Crataegus pinnatifida. Their structures were determined via extensive spectroscopic analyses. Gauge-independent atomic orbital (GIAO) NMR chemical shift calculations, combined with the advanced statistical method DP4+ were employed to establish the relative configurations of four compounds. Next, chiral separation was accomplished by chiral chromatographic column and the absolute configurations of the four compounds were unambiguously assigned by comparison between their experimental electronic circular dichroism curves with the quantum-mechanically calculated curves based on time-dependent density functional theory (TDDFT). All the isolates were evaluated for their neuroprotective activities against H2O2-induced cell injury in human neuroblastoma SH-SY5Y cells. The results showed that two pairs of enantiomers 1a/1b and 2a/2b displayed diff ;erent effect on neuroprotective activity. Among them, compound 2a displayed the most potent neuroprotective effect. Further flow cytometry analysis indicated that 2a could protect SH-SY5Y cells from oxidative damage through inhibiting cell apoptosis.